Coniferyl alcohol polymerization

According to a widely accepted concept, lignin [8068-00-6] may be defined as an amorphous, polyphenoHc material arising from enzymatic dehydrogenative polymerization of three phenylpropanoid monomers, namely, coniferyl alcohol [485-35-5] (2), sinapyl alcohol [537-35-7] (3), and /)-coumaryl alcohol (1). 

As described above, the enzymatic polymerization of phenols was often carried out in a mixture of a water-miscible organic solvent and a buffer. By adding 2,6-di-0-methyl-(3-cyclodextrin (DM-(3-CD), the enzymatic polymerization of water-insoluble m-substituted phenols proceeded in buffer. The water-soluble complex of the monomer and DM-(3-CD was formed and was polymerized by HRP to give a soluble polymer. In the case of phenol, the polymerization took place in the presence of 2,6-di-O-methyl-a-cyclodextrin (DM-a-CD) in a buffer. Only a catalytic amount of DM-a-CD was necessary to induce the polymerization efficiently. Coniferyl alcohol was oxidatively polymerized in the presence of a-CD in an aqueous solution. ... 

Foumand, D. Cathala, B. Lapierre, C. Initial steps of the peroxidase-catalyzed polymerization of coniferyl alcohol and/or sinapyl aldehyde capillary zone electrophoresis study of pH effect. Phytochemistry 2003, 62, 139-146. 

Since the oxidative polymerization of phenols is the industrial process used to produce poly(phenyleneoxide)s (Scheme 4), the application of polymer catalysts may well be of interest. Furthermore, enzymic, oxidative polymerization of phenols is an important pathway in biosynthesis. For example, black pigment of animal kingdom "melanin" is the polymeric product of 2,6-dihydroxyindole which is the oxidative product of tyrosine, catalyzed by copper enzyme "tyrosinase". In plants "lignin" is the natural polymer of phenols, such as coniferyl alcohol 2 and sinapyl alcohol 3. Tyrosinase contains four Cu ions in cataly-tically active site which are considered to act cooperatively. These Cu ions are presumed to be surrounded by the non-polar apoprotein, and their reactivities in substitution and redox reactions are controlled by the environmental protein. 

Moreover, no NMR spectral changes were detected as a consequence of treating dehydropolymerizates from [1- C], [2- C] and [3- C]coniferyl alcohol, respectively, or a dehydrogenative copolymer of /7-[/ing-4 -i C]coumaryl alcohol and coniferyl alcohol, at pH 3.0 with racellular P, chrysosporium culture fluid, or purifled lignin peroxidase, in the presence of H2O2 nor was the outcome affected by prior methylation of the substrates (52). Thus the result originally encountered with the purified spruce wood extract (13) is not representative of polymeric lignin-like preparations at all. 

Figure 1. Removal of 3H at position 5 of the guaiacyl ring of coniferyl alcohol (I) by formation of ring substituted structures (V, VI, VII) during dehydrogenative polymerization.

Figure 2. Dehydrogenative polymerization of a mixture of p-coumaryl alcohol-[ring-2-3H] and coniferyl alcohol-[U-14C], and nitrobenzene oxidation of the DHP to give p-hydroxybenzaldehyde-[ring-2-3H] and vanillin-[formyl-14C].

Figure 1. Formation of guaiacyl lignin and lignin-carbohydrate complexes (LCC) via dehydrogenative polymerization of coniferyl alcohol.

Related research has been reported by Elder and Worley (39), in which MNDO was used to examine the structure of coniferyl alcohol, and its corresponding phenolate anion and free radical. This method represents an improvement over the PPP method, in that MNDO is an all-electron technique, and performs geometry optimizations. It was found that the calculated spin densities and charge values for the reactive sites did not correlate quantitatively with observed bond frequency, but it was observed that positions with partial negative charge and positive spin densities are the positions through which the polymerization has been found to occur. 

Already in 1897 Klason [52] suggested that gymnosperm lignin is derived from coniferyl alcohol. On the basis of experiments using isoeugenol as a model substance, Erdtman [53] advanced the hypothesis that lignin is a product of the oxidative polymerization of coniferyl alcohol ... 

T ignin is one of the most abundant natural products constituting about one-fourth of the woody tissue in plants. Nature has chosen a unique synthetic technique to prepare this cross-linked polymeric material from coniferyl alcohol and related substances. The mechanism of lignin formation is not completely known yet, and the structural characterization of lignin has been only partially successful despite considerable research. 

After polymerization the different lignin subunits are referred to as / -hyrdoxyphcnyl (H), guaiacyl (G), and syringyl (S) residues, depending on whether they originated from />coumaryl alcohol, coniferyl alcohol, or sinapyl alcohol, respectively. 

Guan, S. Y., Mylnar, J., and Sarkanen, S., 1997, Dehydrogenative polymerization of coniferyl alcohol on macromolecular lignin templates,... 

Oxidative coupling polymerization provides great utility for the synthesis of high-performance polymers. Oxidative polymerization is also observed in vivo as important biosynthetic processes that, when catalyzed by metalloenzymes, proceed smoothly under an air atmosphere at room temperature. For example, lignin, which composes 30% of wood tissue, is produced by the oxidative polymerization of coniferyl alcohol catalyzed by laccase, an enzyme containing a copper complex as a reactive center. Tyrosine is an a-amino acid and is oxidatively polymerized by tyrosinase (Cu enzyme) to melanin, the black pigment in animals. These reactions proceed efficiently at room temperature in the presence of 02 by means of catalysis by metalloenzymes. Oxidative polymerization is observed in vivo as an important biosynthetic process that proceeds efficiently by oxidases. 

The network structure of lignin, which is made of phenol units, coagulates the cell wall in wood tissue, which is composed of cellulose and hemicellulose. Lignin is currently a waste product because of its complicated structure [1-4], It is produced by an oxidative polymerization of coniferyl alcohol, sinapil alcohol, and cumarol alcohol (Figure 1) catalyzed by metalloenzymes such as laccase and peroxidases. Laccase is a protein whose active center contains four coppers per one subunit [5-20],... 

Adler and Freudenberg reported a detailed study [21] of the polymerization of coniferyl alcohol with metalloenzymes. Coniferyl alcohol is activated by metal ions to form a radical that can be represented by four resonance states (Eq. 4) and dimerizes by coupling. In the early stage of the reaction, the formation of... 

The control of the coupling of coniferyl alcohol has been attempted. When the metalloenzyme is mixed with coniferyl alcohol, a stepwise polymerization proceeds by dimerization and tetramerization, which result in the uncontrolled couplings. When the monomer is slowly supplied to the enzyme-containing reaction mixture, the radicals attack the terminal end of the polymer chain, because the diluted radical reacts preferentially to form the dimer by the coupling that means that comblike lignin grows on the cell wall. Some attempts have been reported to control the polymerization in the presence of cellulose as the matrix [23,24],... 

Fig. 4-7. Endwise polymerization (Adler, 1977). A guaiacylglycerol-/3-aryl ether structure (1) is dehydrogenated and after resonance, radical c is coupled with a coniferyl alcohol radical b (cf. Fig. 4-4). The /3-5 coupling product (3) is tautomerized and undergoes intramolecular ring closure (a phenylcoumaran structure, 5).

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